The present disclosure generally relates to eye tracking in head mounted display, and specifically to eye tracking solutions utilizing an off-axis camera in a head mounted display with a pancake lens assembly.
A virtual reality head mounted display (HMD) can be used to simulate virtual environments. For example, stereoscopic images can be displayed on an electronic display inside the HMD to simulate the illusion of depth and head tracking sensors can be used to estimate what portion of the virtual environment is being viewed by the user. Some HMDs utilize a pancake lens assembly to direct light of a virtual scene from the electronic display to an exit pupil of the HMD and eventually to the eye of a user. A pancake lens assembly or pancake lens block for a HMD, in one embodiment, includes a back curved optical element and a front smaller curved optical element in optical series. The back optical element includes a surface that is partially reflective to allow a portion of light from the display screen to travel through the back optical element, while another portion of light is reflected. The front optical element includes a polarized reflector that reflects light waves with electric fields oriented perpendicular to a polarization direction of the polarized reflector and allows light waves with electric fields parallel to the polarization direction to travel through. The pancake lens block additionally includes one or more waveplates or other optical elements with the back curved optical element and the front curved optical element that alter the polarization state of light waves travelling through the waveplates. Unlike the optics blocks of other HMDs that do not make use of a pancake lens block, the back focal plane distance of the back curved optical element is relatively small (e.g., ˜5-6 mm). This limits the placement of an eye tracking assembly that have conventionally used a “hot mirror” (or dichroic) between the lens and the electronic display.